1. Field of the Invention
This invention relates to color filters used in image displays and image sensing systems, other image processing systems and color image display apparatuses, and in particular relates to improvements on an availability of light in reflecting type color image display apparatuses.
2. Description of the Related Arts
Recently, there arise many demands on projection type display apparatuses for such as an outdoor public use and a commercial use and for displaying a high definition image on a screen as a largely magnified image.
Generally, the projection type display apparatuses are largely categorized into two types, a transmission type and a reflection type, however, the both types employ a spatial light modulation section having an LCD (Liquid Crystal Display) panel, wherein an incident light beam is allowed to impinge on the LCD panel and an output light beam therefrom is obtained therefrom as a projected light by modulating the incident light by the spatial light modulation section corresponding to picture signals.
Here, the LCD panel is composed of an active matrix substrate.
On the active matrix substrate, there are provided switching elements such as thin film transistors, a picture element electrode layer having a plurality of picture element electrodes, a common electrode layer and a liquid crystal layer interposed between the picture element electrode layer and the common electrode layer. An alignment of moleculars of the liquid crystal layer is controlled by controlling electric potential between the common and picture element electrode layers by the switching elements corresponding to the applied picture signal.
Thus, a read light is modulated responsive to a change of the molecular alignment of the liquid crystal caused by the electric potential distribution between the picture element electrodes and the common electrode layer corresponding to the picture signal applied across the 2 layer of electrodes.
One of the differences between the transmission type and reflection type is that in the former the projection light passes through the transparent substrate of the active matrix, on the other hand, in the latter the projection light is reflected by the picture element electrodes or a dielectric mirror layer provided thereon.
Generally, in the reflection type projection apparatus, there is no need to provide black stripes. This allows to each crystal cell to have a larger aperture ratio. Further, there is few heat generation caused by an absorption of the read light. This allows a larger light output to irradiate the LCD panel for a given power. Thus, a brighter image can be obtained in the reflection type compared with that of the transmission type.
In conventional transmission type projection color image display apparatus, a color image is obtained by using three sheets of transmission type LCD panels corresponding to the three primary colors (R (red), G (green), B (blue)) and a three color composite optical system for composing a color composite light from the three color lights passing through the transmission type LCD, resulting in a large sized apparatus and a high production cost.
As a countermeasure, there is proposed an apparatus employing a single plate type color filter, wherein color filters are disposed as a single layer pattern of stripe, mosaic or a delta state opposing to the transparent picture element electrodes to be driven corresponding to color pattern of the filters. Here, filter elements of the three colors are closely grouped and disposed orderly.
The lights outputted from the filter elements of three colors are visually perceived as a single composite color picture element.
However, in the apparatus, among the read light (a white light) passing through the LCD and impinging on the color filter, only one primary color of the three primary ones is utilized and the rests are excluded, resulting in a low availability of the light. The reason is as follows:
1) When a white light beams is irradiated on the transmitting LCD panel as a read light, a light beam corresponding to a certain color among the three colors only passes through a color filter corresponding to the certain color. Thus, an amount of about 2/3 of the white light beam is not utilized. Further, a transmittance of the color filter is so small that the efficiency of light beam availability becomes much smaller. PA0 2) The transmission type LCD panel is provided with a black matrix surrounding cells as picture elements. Thus, a aperture ratio is small, resulting in a low availability of a light beam because the light beam irradiated on the black matrix is wasted.
As a countermeasure, in Japanese Laid-open Patent Publication 6-308332/1994, there is disclosed a color filter employing a transmissive hologram for a spatial light modulation section used in a transmission type color image display apparatus.
FIG. 27 is a partial schematic side view for explaining operations of a spatial light modulation section in the prior art.
In FIG. 27, a reference character 51 denotes an LCD panel having a plurality of R, G, B-transparent picture element electrodes 51r, 51g, 51b. A reference character 52 denotes a color filter composed of a transmissive hologram including a plurality of unit holograms 52p for spectrally diffracting a read light into respective 3-color lights.
The separated 3-color lights diffracted by the color filter 52 are converged on the plurality of R, G, B-transparent picture element electrodes 51r, 51g, 51b correspondingly with the 3-colors.
According to this prior art, it is possible to utilize almost all the read light by diffracting the read light at a different angle corresponding to the 3-colors with the plurality of unit holograms 52.
As another countermeasure, in Japanese Laid-open Patent Publication 2-500937/1990 corresponding to U.S. Pat. No. 4,807,978, there is disclosed a reflection type spatial light modulation section together with the transmission type color image display apparatus, wherein the transmissive hologram of the color filter is composed of three holographic lens array layers.
FIG. 28 is a partial schematic side view for explaining a function of a spatial light modulation section in the prior art.
In FIG. 28, a reference character 61 denotes a color filter having R, G, B-holographic lens array layers 61r, 61g, 61b, and 62 a glass substrate and 63 a reflective LCD panel.
The reflective LCD panel is composed of a transparent common electrode 64, a liquid crystal layer 65, a reflecting layer 66 and a picture element electrode layer 67 on which a plurality of R, G, B-picture element electrodes 67r, 67g, 67b are disposed. To each of the plurality of R, G, B-picture element electrodes 67r, 67g, 67b, an electric potential is applied from a back side of the LCD panel by a scanning electron beam or a controlling light beam.
The color filter 61 is composed of the R-holographic lens array 61r for exclusively diffracting the R-color light the G-holographic lens array 61g for exclusively diffracting the G-color light, the B-holographic lens array 61b for exclusively diffracting the B-color light in a laminated structure. As shown in FIG. 28, a pitch of the holographic lenses is three times as large as that of the picture element electrodes.
In this spatial light modulation section, a read light (or an incident light) is irradiated on the color filter 61 so that each of quasi-holographic lenses of the color filter 61 diffracts a color light and converges it on a corresponding picture element electrode disposed on an optical axis of the quasi-holographic lenses.
The quasi-holographic lenses are partially superimposed to each other in the construction, however, each of the quasi-holographic lenses exclusively diffracts the corresponding color light. Specifically, the R-holographic lens array only diffracts the R-color light and allows to pass through the rests of the G, B-color lights, the G-holographic lens array only diffracts the G-color light and allows to pass through the rests of the R, B-color lights, and the B-holographic lens array only diffracts the B-color light and allows to pass through the rests of the R, G-color lights.
As a result, the R, G, B-color lights diffracted by the R, G, B-holographic lens array layers 61r, 61g, 61b impinge on the liquid crystal layer 65, and are respectively reflected by the reflecting layer 66 corresponding to R, G, B-picture element electrodes 67r, 67g, 67b. Thus, they impinge again on the R, G, B-holographic lens array layers 61r, 61g, 61b by being modulated by the liquid crystal layer 65 on their ways between the reflecting layer 66 and the R, G, B-holographic lens array layers 61r, 61g, 61b, and the modulated color lights are diffracted again by R, G, B-holographic lens array layers 61r, 61g, 61b to be outputted therefrom to a direction of a light source of the read light.
Generally, in order to increase a diffraction efficiency (a ratio of an intensity of one order diffraction light to an intensity of the incident light) of a hologram, it is necessary to increase a bend angle defined as an angle between the one order diffraction light and the incident light.
Thus, in the Japanese Laid-open Publication 2-500937/60, upon producing the quasi-hologram lenses of the color filter 61, an angle .theta. between a reference light (an incident light) and an object light (diffracted light) are made to be large, wherein the angle .theta. is equal to an incident angle of the reference light.
However, in the prior art, the modulated R, G, B-color lights are returned to the direction of the light source of the read light as mentioned in the foregoing. For this reason, although there is not disclosed in the Japanese Laid-open Publication 2-500937/60, it will be necessary to provide a polarization beam splitter to separate the projection light from the read light in an incident optical system if this color filter is applied to a color image display apparatus. In other words, the read light is allowed to impinge on the color filter 61 through the polarization beam splitter, and the modulated read light (projection light) from the spatial light modulation section impinging again on the color filter is outputted therefrom and separated by the polarization abeam splitter. The provision of the polarization beam splitter in the incident optical system causes reductions of a contrast ratio and availability of light, and a problem of a high production cost because the polarization beam splitter itself is very expensive.
On the other hand, it is well known that the smaller the bend angle of the hologram mentioned in the foregoing becomes, the lager a difference of diffraction efficiency between a P-polarized component (a light component having a plane of vibration parallel to an incident plane of an incident light) and an S-polarized component (a light component having a plane of vibration vertical to the incident plane thereof) of an incident light becomes.
Generally, the diffraction efficiency ".eta." of the transmissive hologram depends on an amount of modulation ".DELTA.n" of refractive index and a thickness thereof "t" and an incident angle ".theta.". When the incident angle ".theta." is set up as large as 60.degree. to 90.degree., a diffraction efficiency ".eta.p" of the P-polarized component and a diffraction efficiency ".eta.s" of the S-polarized component respectively are apt to behave a periodic change with respect to changes represented by a function F (.DELTA.n, t) as shown in FIG. 29.
FIG. 29 is a graph showing a diffraction efficiency ".eta.p" of the P-polarized component and a diffraction efficiency ".eta.s" of the S-polarized component with respect to changes represented by a function F (.DELTA.n, t).
And, under a condition that both the ".DELTA.n" and "t" are constant, when the incident angle ".theta." is decreased as small as 0.degree., the characteristic of the diffraction efficiency ".eta.p" of the P-polarized component and that of the diffraction efficiency ".eta.s" of the S-polarized component has a similar periodic characteristic, resulting in the same one in principle when ".theta."=0.
Accordingly, in the changes of the diffraction efficiency of the ".eta.p" and ".eta.s" shown in FIG. 29, it is possible to allow the ".eta.p" of the P-polarized component to be, for instance, about 18% under a condition that the ".eta.s" of the S-polarized component is held to be a maximum of 100% by setting up the incident angle ".theta." to be 75.degree..
In this regard, the inventors of the present invention has devised a useful color filter applicable to the color image display apparatuses capable of displaying the composite color image with a high contrast ratio and a high availability of light without providing the polarization beam splitter.